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. Author manuscript; available in PMC: 2010 Mar 30.
Published in final edited form as: Trends Genet. 2007 Apr 18;23(6):284–292. doi: 10.1016/j.tig.2007.03.018

Figure 4.

Figure 4

Higher order chromatin silencing models. (a) Transcription-based looping model: ncRNA transcription and processing is proposed to induce looping of the DNA through interactions between its 5′ and 3′ processed ends. The induced chromatin loop physically isolates a domain activator needed for expression of all imprinted protein-coding genes in the cluster. In this model, the sequence of the ncRNA is not important for silencing function. Thus, this model would also fit the category of transcription-based models. (b) ncRNA-induced compartment model, based on the ncRNA forming a compartment that does not favour mRNA transcription. This could occur for many reasons: for example, the ncRNA physically excludes access to a transcription-compartment, or it could act as a sink to sequester RNA polymerase II, or it could create a subnuclear compartment that favours unspliced, non-exported RNAs.